Investigations On Mesoscale Structure In Gas-solid Fluidization And Heterogeneous Drag Model

For the understanding of the comprehensive, mutli-scale and heterogeneousgas-solid fluidized flow and transfer process, and for the improvement of thenumerical simulation method for large fluidization systems, an investigationwith the flow heterogeneity as the principle line was conducted on themeso-scale structure (cluster) and its effects on the flow and transfer process.This research aims to improve the Energy Minimization Multi-scale (EMMS)theory, develop a drag model with high accuracy and condition universality, andcombine with the Euerlian-Eulerian approach for simulating the heterogeneousgas-solid fluidized flows.In-depth analysis was conducted on the EMMS theory. The results indicatethat the direction for the theory improvement is the supplement of basicequations. Sensitivity analysis was carried out to investigate the influence ofmeso-scale structure or cluster characteristics on the drag function. The resultsshow that, the cluster density (εsc) rather than the cluster diameter (dcl) plays thecrucial role.The relationship of the cluster characteristics, especially the cluster density,and the local time-averaged solid concentration (εs) was investigated. The localheterogeneous index which describes the deviation degree from the uniformflow state was found to reach maximum at the εsof0.15and tend to zero at bothlarge and small εs. The cluster diameter equation in EMMS theory was modifiedwith the local heterogeneous index. The straight line with a slope of one (εsc=εs)distinguishing the heterogenous and uniform flow state and the horizontal linewith a slope of zero (εsc=εsmf) representing the most hetergeneous flow statewere proposed to determine the variation range of cluster density. Then, themathematical model describing the relationship of cluster density and εswasproposed and validated by experimental data.For the universal applicability of the above-proposed cluster model forvarious fluidization conditions, investigations were conducted on the changes ofthe overall flow heterogeneity with various flow patterns. With the increasinggas velocity, the flow pattern experiences a “bubbling fluidizaiton-fast fluidization-pneumatic transport” process which corrspondes to a“uniform-heterogeneous-uniform” process. The integral gas-solid slip velocityrepresenting the overall flow heterogeneity also firstly inceases and thendecreases with the increasing gas velocity. Combining the general fluidizationdiagram and the integral slip velocity diagram, the dimensionless parameter Re*was proposed to represent various operating conditions.The factor Ψ which represents the relationship of the cluster density andoperating conditions was proposed to decide the deviation extent from theuniform state. The mathematical relationship of Ψ and Re*was established andverified by experimental data. In this way, the cluster model is universallyapplicable for various fluidization conditions.The EMMS theory was improved with the cluster model and then a dragmodel for heterogeneous gas-solid fluidized flows was developed and calledQC-EMMS. The drag model was verified to reveal the essence of drag reductionin heterogeneous fluidization systems on both the grid and system scale.Combining the QC-EMMS drag model with Eulerian-Eulerian approach,numerical simulations were carried out for the hetergeneous gas-solid fluidizedflows under various operating conditions, including Geldart A、B paritcles,2D、3D beds, different solid fluxes and different system sizes. The predicted flowparameters agree well with experimental results and the simualtion errors aremuch smaller than other drag models.